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    Re: AP terminology, WAS: 2-Body Fix -- take three
    From: Jeremy C
    Date: 2009 Nov 12, 10:27 EST
    A DR is a position based on course and speed.  An AP is a position used for calculations. 
    I never use my DR for the AP for navigation at sea.  When doing 249/229 reductions I use an AP chosen for each sight to give me whole degrees of Latitude and LHA.  If I am using a calculator or computer, I choose a convenient GPS fix (nearest half hour) as the AP. 
    The only time I can think of that you wouldn't need an AP is when scribing circles of equal altitude because you are not really doing any trig computations.  This can be done on computer easily enough, and in certain circumstances on paper, but without an idea of where you are in the world, you need at least 3 LOP's to deal with the fact that two circles will offer two intersections.  When shooting high-altitude circles, having a DR still may not help as I have seen several times when shooting them.
    In a message dated 11/11/2009 11:01:13 P.M. Eastern Standard Time, jhkarl@att.net writes:

    No, Peter, I am not.

    The term AP is the location used to solve the navigation triangle in
    the St. Hilaire method.  This location might be the DR, it might be an
    EP (estimated position), it might be a recent fix of any kind, it
    might be a GPS position (used to check the GPS, of course).  Again,
    the DR is a particular choice for the AP.  And there should be no
    confusion between the AP and a DR.

    John --~--~---------~--~----~------------~-------~--~----~
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